Mechanism of casing deformation in the Changning-Weiyuan national shale gas demonstration area and countermeasures

Casing deformation occurs during the development of Changning-Weiyuan national shale gas demonstration Area in the Sichuan Basin. In view of this, the correlation between the casing deformation and the geological characteristics as well as hydraulic fracturing was analyzed. It is shown that fracture and bedding (hereinafter collectively referred to as fractures) in faults are the internal cause for casing deformation and hydraulic fracturing is the external cause. Then, the mechanism of casing deformation was clarified. As the fracturing fluid flows along a certain passage into the natural fracture, the pore pressure in the fracture rises. When the critical pressure is reached, the natural fracture is activated to move, and consequently casing deformation occurs. Fluid may flow along three pathways, i.e., the hydraulic fracture, the axial fracture along the borehole axis created during hydraulic fracturing and the micro-annulus of cement sheath induced by repeated fracturing. Finally, some specific preventive measures were put forward. First, install packers in the whole sections with fractures and beddings. Second, improve the properties of slurry to avoid the formation of micro-annulus of cement sheath. And third, adopt the fracturing technologies to avoid the casing from repeated high pressure. The study results provide the guidance for the solution to shale gas casing deformation.